The launch is the first stage in the Cluster II programme, which could provide scientists with a better understanding of so-called "space weather", which in some cases can knock out communication satellites orbiting the Earth.

The Esa has been dogged by difficulties in its efforts to study the weather in space. The original Cluster I programme ended in disaster in 1996 when the Ariane 5 rocket carrying its satellites blew up 40 seconds after take-off.

The launch of the two Cluster II satellites was aborted on Saturday because of a connection problem on the ground, between the launch pad and the rocket launcher, according to Esa officials.

Solar flares

Like its predecessor, the Cluster II programme comprises four satellites - except that they are being launched in pairs on two separate Soyuz rockets.

The second rocket is scheduled to blast off from the Baikonur cosmodrome in Kazakhstan on 9 August.

The rocket is carrying two Cluster satellites

The four satellites' target area is the Earth's magnetic environment - the magnetosphere.

The magnetosphere protects life on Earth from the stream of charged solar particles - electrons and protons - that are blasted continuously from the Sun.

Cluster will collect data on how the magnetosphere interacts with this solar wind and with more violent solar events such as flares and so-called coronal mass ejections (CMEs).

Scientists warned on Friday that one of these solar flares had just burst from the Sun's surface, ejecting billions of tonnes of charged particles into space.

A booster will put the satellites into their elliptical-shaped orbit

Some of them are heading towards Earth and are expected to cause a geomagnetic storm.

Scientists are fascinated by these "space weather" phenomena.

The charged particles, which travel at hundreds of kilometres a second, can knock out electronics on satellites and in extreme cases interfere with power grids on the Earth's surface.

Armed with data from Cluster II, scientists hope that they will be able to predict such threats to satellites with more certainty.

Northern lights

Early in the mission, the spacecraft will spend most of their time flying on the side of the Earth that faces away from the Sun.

After six months, they will move in front of the planet to investigate the polar cusps.

These are weak points in the Earth's magnetic shield where charged particles penetrate the upper atmosphere and generate the spectacular Northern and Southern Lights.

From the end of December, data will be coming down from the four satellites at a rate of one gigabyte (two compact disks) every day.